Pluto is now the most distant object ever visited by humanity

Pluto fills the frame in an image from NASA's New Horizons spacecraft taken July 13, when the spacecraft was 476,000 miles from the dwarf planet. (NASA EPA)

After a nearly decadelong journey, the New Horizons spacecraft zoomed past Pluto early Tuesday. The dwarf planet is now the most distant object ever visited by humanity.

New Horizons' closest approach was scheduled for 4:49 a.m. PDT when the spacecraft would be just 7,800 miles away — close enough to take the first high-resolution images of Pluto's mottled landscape.

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But Tuesday morning NASA officials said it was likely that New Horizons arrived at its destination 72 seconds earlier than projected.

That still puts New Horizons solidly in its target window. It also means the mission got about 43 miles closer to the dwarf planet than originally planned, said New Horizons principal investigator Alan Stern.

A small batch of images is expected to be released Thursday or Friday.

Because of the high speeds necessary to send the craft to the Kuiper belt, it was unable to orbit or otherwise linger near Pluto or its five moons. Instead, New Horizons flew by at 30,800 mph.

To maximize its scientific data collection, the spacecraft is not scheduled to communicate its status until Tuesday evening.

"We lost the signal as planned at 11:17 p.m. Eastern and there is nothing we could do but trust we had prepared it well to set off on its journey on its own," said Alice Bowman, New Horizons' mission operations manager.

After the spacecraft went quiet several members of the New Horizons team remained in the operations room anyway.

"Even though we knew it wasn't going to be talking to us, we wanted to be there so that we would be with it while it went through that journey," Bowman said.

The brief encounter was a product of the realities of orbital physics, which precluded any chance to slow down the spacecraft.

Pluto is two-thirds the size of Earth's moon, and its gravity is very weak — just one-fifteenth of Earth's gravity.

It's easy to get caught in the gravitational pull of a gas giant like Jupiter, which is about 300 times the mass of Earth. If you can get a spacecraft near enough, the planet's powerful gravity would draw it down until it could settle into a stable orbit.

But with Pluto, the dynamics are different. A spacecraft could only get caught in an orbit around Pluto if it were moving slowly, at nearly the same velocity as the dwarf planet.

It is possible in theory to slow the spacecraft down ahead of its rendezvous. But that would have required New Horizons to launch with almost 70,000 pounds of fuel, according to calculations by Ben Montet, an astronomy graduate student at Caltech.

Considering the spacecraft weighed just over 1,000 pounds when it left Earth in 2006, that was not going to happen.

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And because this mission is so long, and has taken the spacecraft so deep into the solar system, mission architects felt the simplest — and most cost-effective — plan was to blow past Pluto without stopping.

So, scientists had to make due with a fast flyby, carefully choreographing the observations of New Horizons' suite of instruments to collect as much information as possible in a short amount of time.

The budget for the mission was a relatively low $700 million. It is managed by the Johns Hopkins Applied Physics Laboratory in Maryland, which designed, built and operates the spacecraft.

At the time of the launch, Pluto was still considered a full planet. It was demoted a few months later in 2006 because the International Astronomical Union said it was not hefty enough to prevent similar-sized objects from forming in its section of the solar system.

As New Horizons has gotten closer to Pluto, its instruments already have revealed new facets of the dwarf planet.

Over the weekend, scientists discovered that it is a bit bigger than they expected. Previous estimates had put it somewhere between 715 and 746 miles, but new information suggests Pluto's radius is about 736 miles across, putting it solidly at the upper end of the estimate.

The measurement suggests that Pluto's interior is less dense with more ice and less rock than was previously thought, said Stern.

At a news conference Tuesday he noted that it appears there is snow on Pluto.

But the New Horizons team is not saying what the new data might mean until they've been able to study it.

"Especially when we are exploring brand-new planets, science on the fly usually turns out to be wrong," Stern said.

Pluto's icy polar cap appears to be made of methane ice and nitrogen ice, and images collected by the spacecraft show strange dark poles on Pluto's Texas-sized moon, Charon — something that has never been seen before in the solar system.

"The system is enchanting in its strangeness and alien beauty, showing us complex and nuanced surfaces that are beyond our wildest dreams of science," Stern said.

There is, however, a slight risk of calamity.

Computer models suggest there is a 1-in-10,000 chance that a rogue piece of space dust, perhaps from a recent impact on one of Pluto's satellites, could damage the spacecraft or its instruments.

Although the odds are low, mission scientists can't rule it out. "You always take on a risk when you are flying into the unknown," Stern said.

If all goes as planned, the spacecraft will break from its scientific observations at 1:30 p.m. PDT to send word across 3.5 billion miles of space that it has sailed through the Pluto system unscathed.

That message, moving at the speed of light, will take 4.5 hours to get to scientists on Earth. It is expected to arrive at 6:02 p.m.

During its brief window of intense scientific data collection, New Horizons' seven instruments are scheduled to gather information that will be studied for years to come, including measurements that will reveal the height of Pluto's mountains, the depths of its valleys and the temperature of its surface.

In the hours after its closest approach, the spacecraft will turn around and point its large antennae toward Earth to receive a radio signal sent by NASA's Deep Space Network through Pluto's atmosphere. This will help scientists determine what it is made of, and whether it shares an atmosphere with Charon.

Cathy Olkin, deputy project scientist for the mission, said it would take nearly 16 months to get all the data from New Horizons back to Earth.

There are plans to get more mileage out of the trip as well, including a visit to another object in the Kuiper belt sometime in the future.

Decisions about extending missions aren't made until the primary mission is complete, but in the fall of 2014, scientists identified three additional bodies that would be worth a visit.

These icy worlds are significantly smaller than Pluto. Scientists believe they are frozen relics from the dawn of the solar system, and could teach us more about our origins and the diversity of objects in this little-known region.

Each one is about a billion miles from Pluto, and it would take until 2019 to get there.

Twitter: @DeborahNetburn

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